Chapter 34.
Triggers

This chapter provides general information about writing
trigger functions. Trigger functions can be written in most of
the available procedural languages, including PL/pgSQL (Chapter
37), PL/Tcl (Chapter 38), PL/Perl (Chapter
39), and PL/Python (Chapter 40). After reading this chapter, you
should consult the chapter for your favorite procedural language
to find out the language-specific details of writing a trigger in
it.

It is also possible to write a trigger function in C, although
most people find it easier to use one of the procedural
languages. It is not currently possible to write a trigger
function in the plain SQL function language.